EQ differences using ASCE and UBC

[m1208]

I have a two story wood structure with wood floor/roof and wall plywood construction. I get very different seismic coefficient using the ASCE and UBC; which does not seem to be reasonable.

For ASCE equation V=(F Sds/R)W= 1.1*0.267/6.5 W = 0.045W

For UBC equation V=(Cv I/R T)W = (0.54*1.0/5.5*0.15) W = 0.65 W

For UBC equation Vmax=(2.5 Ca I/R)W = (2.5*0.36/5.5)W= 0.16 W; which I can divide by 1.4 to convert it to allowable stress design to get 0.12W.

My question is why the coefficient of 0.045W is so low with for ASCE design compared to 0.12W using the UBC? Is there any minimum limit for the seismic coefficient using the ASCM?

Thanks

 

[csd72]

As all earthquake loads are approximations and research is increasing the understanding of things at a rapid rate there is no surprise as to the variability between codes.

All you can do is ensure that you comply with the code under the relevant jurisdiction.

 

[JoshPlumSE]

Some thoughts:

1) You'll have about a 15% difference between them based on the different R values. That's not even a difference in the code, but rather a difference in a variable.

2) The shape of the UBC and ASCE acceleration curves is essentially the same. Where Cv = Sd1, and Sds = 2.5*Ca. In your case 2.5*Ca = three times the Sds value!! This is the main cause of your difference. This may be a case where the seismic maps were drammatically changed between 1997 and today. Perhaps to reflect new knowledge about a previously unkown (or under estimated) fault.